Why Safari fails to load or run properly in Safari can be complicated. I have had success "fixing" balky sites by turning off extensions. Turning off extensions in older versions of Safari was on the standard menu bar but in Safari 9 that has been moved to the Develop Menu Ryck mentioned previously.
If turning off the extensions works, you can disable them one at a time in Safari > Preferences > Extensionsons until the offender is identified.
The settings for Cookies can also cause problems with a few web sites.
